The overall objective of this study is to contribute to the understanding of the association between chronic occupational exposure to low dose, penetrating ionizing radiation and cancer mortality. The cohort of workers employed at Oak Ridge National Laboratory provides a valuable resource for studying radiation health effects; however, inconsistent estimates of the radiation-cancer dose-response relationship reported in previous studies of this cohort suggest the need to further examine time- related confounders and effect modifiers. The proposed study will describe relationships between low dose radiation and mortality using an expanded cohort of ORNL workers, extended mortality follow-up, including workers with employment at other DOE facilities, and recently revised estimates of radiation exposure. In evaluating radiation-cancer dose- response relationships in this updated cohort, analyses will consider time-related confounders and effect modifiers which have been suggested by previous occupational studies. These time-related factors include time patterns in radiation exposure and covariates, selection in and out of employment, changes over time in the quality of exposure measurement, contribution of age at exposure, exposure lag, and time since exposure. Recently developed analytical methods for studying induction time will allow investigation of potentially critical time windows of exposure. Modification of the dose-response pattern by age at exposure will be also investigated. Finally, evidence for residual confounding will be sought by considering the association between radiation and non-cancer mortality. This study will provide new epidemiological data about low dose radiation health effects from one of the best available data sources. Furthermore, the analyses of induction time and modification by age at exposure will provide new insights into the radiation-cancer relationship, and expand the methodological literature on time-related factors in occupational epidemiology. Finally, the public concern about low dose radiation, and the prevalence of the exposure make this study a valuable service for addressing these public concerns.